Inhibiting thermal decomposition of alkali metal silicates



Lulu \llllllialJj Diamond Shamrock Corporation, a corporation of A still further object of the invention is to provide a D l new method for inhibiting thermal decomposition of No Drawing. Filed Oct. 8, 1964, Ser. No. 402,607 liquid silicates.

2 Claims. (Cl. 106-74) These and other objects and advantages of the invention will appear more fully hereinafter. Broadly stated, the present invention involves adding 8. ABSTRACT O THE DISCLOSURE small but effective amount of zinc oxide to a silicate, The thermal degradation of aqueous alkali metal silithereby to inhibit decomposition, which decomposition cate solutions is inhibited' by the addition of small typically is evidenced by lncreaslng turbidity and subseamounts, generally, 0.01 to 2.0%, of zinc oxide. qutent formation of an undesirable precipitate in the silt- WMMC: ca 6- I More particularly, the present invention comprises the This invention relates to the inhibition of decomposition addition to a silicate of a decomposition inhibiting amouilt of silicates. I of zinc oxide. In general, the amount of zinc oxide to he Liquid silicates, especially liquidalkali metal silicates, added may vary from about 0.01% to about 2% by and notably aqueous alkali metal silicates such as sodium welght of the slhcate. although In m pp c even silicate solutions, are well known and widely used comlarger amounts, such as up to 20% by weight can bemercial chemicals. The uses to which such silicate solued nh bit decomposition. However, in most instances tions are put are diverse and the problems encountered in there is usuallyno particular advantage insofar as inhibitthe preparation and maintenance of such solutions also mg decomposltlon is concerned in using larger amounts. very id 1 i h h d use intended, Despite many It is presently preferred to add zinc oxide to the silicateadvantages, in some applications problems are encoun- In an fl efa ge o about 0.4% 150 06% tered because of decomposition of the silicate, especially y Weight of h q l l0 alliall metal Silicate. where the silicate is exposed to an elevated temperature P e y hq um sl lcate having an Si0 ,:Na,Q in use or storage. This decomposition, often occurring Weight ratio of at least 2.251, although the invention con} over an extended period of time, although accelerated by templates that the 21116 oxide can be added to a dry silif exposure to elevated temperatures, tends disadvantagee S a a ommercially available hydrated silicate. 1n ously to alter the composition of the silicate, general, the stabilizing cifect of th1s invention is obtained- For example, one of the advantageous uses of liquid 4 bOth m concentrated commercially available liquid sillsilicates, especially aqueous sodium silicate solutions, is hates s Well s further diluted s licates. as metal impregnants for the reduction or elimination of 5 T fe idate as used In the specification and porosity in castings. Castings often contain a considerable claims 15 n ende o Include various water soluble sillamount of localized tiny labyrinthine voids arising from ate lneludmgboth alkah metal silicates and ammonium the escape of entrapped gases within the metal while it is slllcate. Further, the expression alkali metal silicates as: ill l i l fluid d l caused at ti b h i k intended torefer to water soluble silicates of the various of the metal during cooling. There results what can be alliali m a Sodium, Potassium. lithium, rubidium characterized as a liquid permeable interlaced sponge-like a d cesium; 1i Willhe hf s Of course. l Sodium pore network. Such'porosity while generally not impairslllcates P P-[ slllcales are the {Flore y li th structural soundness f a ti d i some able alkal metal silicates and that sodium silicate generiustaucus cause d i bl porosity of h fi i h d i l ally constitutes the common commercial alkali metal sili- It is known that liquid silicates used as impregnants proe For 9 p r icular emphasis hereinafter is vide means for sealing oil this porosity. In many instances given tothrs'nnatenal which constitutes a preferred alkali this can be accomplished simply by dipping the metal etal silicate. X A article into a liquid silicate, such as an aqueous sodium The llqlllil Porlloh of the liquid Silicate generally silicate. In other cases, the metal articles are subjected to most n g ly is Water. In practice, the liquid ,siii more involved treatment steps including exposure, to cate used 6311 b3 a commercially available aqueous Sillvacuum and pressure during impregnation. In practioe, cate, typically containing about 45% to 70% by weight the liquid silicates typically contain small amounts of W However, if desired, the line OXide c also be clay, powdered asbestos and/or metal oxides. In some added to a y Silicate Prior to mixing With W thus instances, dilution and/or wetting agents and peptizing forming a composition embodying the invention. agents are employed to facilitate penetration of the sili- A particularly preferred metal impregnant composition t Th use f various types f metal impreguauts i of this invention is prepared by combining sodium silicate disclosed in Precision Metal Molding, Sept. 1953, p. 114. having all sioziNaao Weight ratio of at least 2111, and In use, it has been found that despite the advantages generally p i 4-011 which Slllcate otherwise is Subject provided by the use of liquid silicates in this manner, to decomposition, especially thermal decomposition, and some practical problems arise because the liquid silicates, about (14% i0 05% y weight M the liquid Silicate of especially aqueous alkali metal silicates, tend to decom- Zinc Oxide having a Particle Size of less than 200 mesh. pose, in time, especially when subjected to elevated tem-' Preferably less than 400 mesh- In general, the ler peratures. While the complete mechanism of this decomthe Particle Size of the c Oxide, the better. position is not understood, it is believed that in the case In the Practice Of this invention, Various commercially of sodium silicates the decomposition involves formation available aqueous Sodium silicates can be p y By United States Patent 0 Accordingly, a principal object of this invention is to 3,410,706 avoid the difiiculties heretofore encountered with decom- INHIBITING THERMAL DECOMPOSITION OF ALKALI METAL SILICATES Cletus E. Peeler, Jr., Painesville, Ohio, asslgnor to of a hard layer or other undesirable precipitate of sodium tetrasilicate or related polysilicates.

position of silicates, especially those used as metal impregnants.

way'of example, the following, with indicated SiO,:Na- O ratios, are available commercially.

TABLE 1 Sillcate N 0. Percent Percent Percent SlOnNmO, Gravit N820 S10: E20 Wt. Ratio "Be.

'- Dry silicate.

A specifically preferred composition of this invention is 71.3% sodium silicate with a SiO,:Na O wt. ratio of 2.84:1 (silicate number 5 from Table 1), 0.5% zinc oxide, and 28.2% added water to give a total solids content of 31.2%.

In order that those skilled in the art may more completely understand the present invention and the preferred methods by which the same may be carried into effect, the following specific examples are offered:

EXAMPLE 1 500 gms. each of silicates 1, 2 and 5 from Table No. 1 plus a special silicate prepared by mixing 250 gms. each of silicates l and 2 are split into two portions of 250 gms. each. Each portion is placed in a .6 pt. metal can and to each can is added 80 gms. of water. This gives a specific gravity of 1.32 which approximates that used in metal sealant compositions. After stirring thoroughly, approximately 80 gms. is discarded from each can and 1 gm. of powdered ZnO is added to one can of each grade silicate with thorough stirring. The 8 samples (4 with ZnO and 4 controls) are sealed and placed in an oven maintained at 120 to 130 F. for thermal decomposition studies. The samples are observed at intervals of 48 hours, 5 days, 2 weeks, 1 month, 6 weeks, 19 weeks, 28 weeks and 45 weeks. Since the only change noticed at the 48 hourand 5 day points is the formation of a small layer of undissolved ZnO. the samples are remixed after these two examinations prior to rescaling and replacing in the oven. At all other examination points the contents are not disturbed more than is required to evaluate any deposits.

At 6 weeks the samples of silicate No. 2 and No. 5 that do not have ZnO. added begin to show a tendency to decompose. At 19 weeks breakdown of these 2 samples is definite with no significant change in the other 6 samples. At 28 weeks a large amount of breakdown is observed with almost 16' of silicate No. 2 solidified. There is still no change in the other samples.

When a final observation is made at 45 weeks the following is noted.

4 Example 2 The following test is run for 3 purposes: (A) to determine stabilization effect on undiluted commercial aqueous sodium silicates, (B) to determine the amount of ZnO necessary to effect stabilization, and (C) to test the eifect of high dispersion of ZnO vs. hand mixing.

Two slurries are prepared as follows:

(A) Silicate No. 7 (See table 1) 992 gms. and 8 gms. of ZnO are thoroughly mixed (15 minutes high shear mixer).

(B) Silicate No. 5 (See table 1) 992 gms. and 8 gms. of ZnO are thoroughly mixed. Eighteen samples are prepared for testing by mixing in 8 oz. steel cans as indicated in the table below. This amount of material in the 8 oz. cans used gives a 1%" depth of silicate solution in the can.

Wt. (gms.) Wt. (gms.) Sample Silicate No. Wt. gms. Percent Silicate No. Wt. (gms.)

7 (Un- Slurry A ZnO 5 (un- Slurry 13 treated) treated) 0.5 gm. of ZnO added by hand mixing.

The samples are hand mixed, sealed, and placed in an oven at 80 C. for thermal decomposition observations.

When the samples are observed after 7 days storage at 80 C. there is no change to be noted visually. The samples remain slightly turbid due to suspended, undissolved ZnO.

By 5 weeks the breakdown has begun and at 5, 6% and 9 weeks it is determined and reported as follows:

[Percent breakdown-height solid/total ht. (1%):100]

Sample 5 weeks 6% weeks 9 weeks Discontinued.

BiOuNazO, See Table 1 Percent Wt. Ratio Silicate No. ZnO

2.0: 1 1 N 0 break down-Trace 0t sedlmen 2.0:L. 1 plus ZnO 0. 4 No break down-Less sediment than above. 8 am N beak d From this table it can be determined that the most M efiective amounts of ZnO are from 0.4 to 0.6%. No sig- Special silicate M No break down-Less nificant advantage appears from the use of high speed plus sediment than dispersion of the ZnO-silicate mixture. 8111mm2 n g g down- It is to be understood that although the invention has 24:1. 21111132110 0,4 g? K53 5 been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes -1 5 H gfiifiillifitfif and alterations therein may be made which are within 5 Z110 g %g the full intended scope of this invention as defined by the From this it can be concluded that sodium silicates having Si0 :Na,O weight ratios of 2.2:1 or less showno breakdown and that ZnO definitely inhibits break down of the higher ratio silicates.

5 6 to said silicate solution from 0.4-2.0 percent by weight 2,536,871 1/1951 Carlton 106-84 of the silicate solution of zinc oxide. 2,680,081 6/1954 Probert et al. 106-74 2. The method of claim 1 wherein the silicate is an 2,998,328 8/1961 Munger et a1 106-84 aqueous sodium silicate. 3,093,493 6/1963 Freyhold 106-74 5 3,100,154 8/1963 Oshima et a]. 106-84 References CW1 3,146,828 9/1964 Mann 106-84 UNITED STATES PATENTS 1,923,769 8/1933 wood 106 84 TOBIAS E. LEVOW, Przmarzy Examiner. 2,111,131 3/1938 Weygandt 106- 84 MOTF, Assistant Exammer- 2,311,271 2/1943 Ware 106-84 10 

